Bug 1571769 - [marionette] Enable Mn jobs with Fission enabled on try. r=ahal

Differential Revision: https://phabricator.services.mozilla.com/D40862

--HG--
extra : moz-landing-system : lando

taskcluster/ci/test/marionette.yml

@@ -30,6 +30,7 @@ job-defaults:
         by-test-platform:
             windows10-aarch64.*: 2
             default: default
+    variants: ["fission"]
 
 marionette:
     description: "Marionette unittest run"